Optical amplifier device

ABSTRACT

The optical amplifier apparatus comprises an optical amplifier ( 3 ) which amplifies an input signal light, an output detecting unit ( 5 ) which detects an output level of the optical amplifier, an output control unit ( 6 ) which controls an output level of the optical amplifier according to an output level detected by the output detecting unit ( 5 ), a gain inclination detecting unit ( 7 ) which detects a gain inclination relating to a wavelength of the optical amplifier, and a gain inclination control unit ( 8 ) which controls a gain inclination of the optical amplifier ( 3 ) according to a gain inclination detected by the gain inclination detecting unit ( 7 ).

TECHNICAL FIELD

[0001] The present invention relates to an optical amplifier apparatus.More particularly, this invention relates to the optical amplifierapparatus which is utilized for an optical repeater and others forwavelength multiplex transmission.

BACKGROUND ART

[0002] In recent years, along the rapid increase in demand forcommunications using the Internet and the like, there have been activelyprogressed researches and developments into and practical application oflarge-capacity and high-speed transmission systems in networks. As oneof such signal transmission systems, a wavelength multiplex transmissionsystem for transmitting signals by multiplexing signal lights of aplurality of mutually different wavelengths through one transmissionfiber has been known. According to signal transmission based on thewavelength multiplex transmission system, it is possible to transmitinformation for each signal of each wavelength, and this system issuitable for large-capacity transmission. An optical fiber amplifier (anoptical amplifier) constructed of a rare-earth-added optical fiber hasthe following characteristics, and is suitable for application towavelength multiplex transmission. The amplifier does not depend on atransmission speed, the amplifier can simplify repeaters, and theamplifier collectively amplifies all input signal lights.

[0003] However, the optical amplifier has gain wavelength dependency,and it has been known that there occurs a variance between wavelengthsin the light output or the gain of each wavelength after theamplification. Therefore, there occurs a variance between wavelengths inthe optical power after the transmission. Particularly, when carryingout a multi-stage repeating by the optical amplifier, the variancebetween wavelengths due to the optical amplifier at each repeating stageis accumulated. This brings about the inconvenience that the variancebetween wavelengths of the optical power after the transmission becomeslarger.

[0004] As an example of an optical amplifier gain equalization techniquefor solving the above problem, there is a technique (an opticalequalization amplifier) as disclosed in Japanese Patent ApplicationLaid-open No. 9-211507. This optical equalization amplifier is anapparatus for equalizing the output levels of a plurality of wavelengthcomponents as well as making constant the light input and output levels,in the amplification of wavelength-multiplexed lights. As shown in FIG.17, the optical equalization amplifier has an optical amplifier 103 anda variable optical attenuator 104 arranged between an input terminal 101and an output terminal 102.

[0005] The optical amplifier 103 has such gain inclinationcharacteristics, that a gain at a long wavelength side becomes smallerwhen the excitation ratio is high, and that a gain at a short wavelengthside becomes smaller when the excitation ratio is low. In other words,the optical amplifier 103 has a negative inclination of gain relative towavelength when the excitation ratio is high, and has a positiveinclination of gain relative to wavelength when the excitation ratio islow. As an example of a means which changes the excitation ratio, thereare an excitation power control unit, and an optical level control unitfor controlling a signal light that is input to the amplifier.

[0006] In the above optical amplifier apparatus, an inputwavelength-multiplexed signal is amplified by the optical amplifier 103.A level of each wavelength is detected at the output side of the opticalamplifier 103. The optical amplifier apparatus controls the excitationpower (gain control) of the optical amplifier 103 so that the levels(gains) of the wavelengths are equalized. An output level is detected atthe output side of the variable optical attenuator 104. The opticalamplifier apparatus controls (output control) the attenuation of theoutput signal light of the variable optical attenuator 104.

[0007] However, a transmission line and a passive device havewavelength-dependency attenuation, and therefore, optical signals ofdifferent levels are received at the input of the optical amplifier.This brings about a variance in the optical levels between thewavelengths. A variance of optical levels generated by one opticalamplifier is small. However, as dozens to hundreds of optical amplifiersare provided for long-distance transmission like a submarine cable thatconnects between continents, the accumulation of such variances resultsin a small optical level in a channel of a specific signal wavelength.This has a risk of deteriorating an optical signal to noise ratio.

[0008] As the wavelength signal of the lowest power among themultiplexed wavelengths becomes a lower limit of reception power afterthe transmission, a maximum transmission distance is limited by thewavelength signal of the lowest power. Therefore, it is important tolower the variance between wavelengths after the transmission, in orderto expand the maximum repetition transmission distance. For minimizingthe variance in the optical signal to noise ratio between channels afterthe transmission and minimizing the variance in the receptioncharacteristics, there is a technique of providing a gain inclination atthe transmission side in advance (pre-emphasizing). An inline automaticgain inclination compensator may be inserted into the transmission line.

[0009] However, the provision of a large pre-emphasis lowers an opticalSNR, and this also distorts a signal waveform due to a nonlinear effectof the transmission line attributable to the rise in signal opticalpower of a distance average. The pre-emphasis has inconvenience that thepre-emphasis effect becomes smaller as the transmission distance becomeslonger. On the other hand, the inline automatic gain inclinationcompensator can minimize excessive deterioration of the optical SNR andthe reception characteristics. However, there are other problems likethe reliability of this compensator and the increase in the cost of theapparatus.

[0010] It is an object of this invention to provide an optical amplifierapparatus capable of controlling a gain inclination and making constanta light output level in amplifying wavelength-multiplexed lights,without depending on the provision of pre-emphasis and withoutgenerating other problems like the reliability of a compensator and theincrease in the cost of the apparatus.

DISCLOSURE OF THE INVENTION

[0011] The optical amplifier apparatus according to one aspect of thepresent invention comprises an optical amplifier which amplifies aninput signal light, an output detecting unit which detects an outputlevel of the optical amplifier, an output control unit which controls anoutput level of the optical amplifier according to an output leveldetected by the output detecting unit, a gain inclination detecting unitwhich detects a gain inclination relating to a wavelength of the opticalamplifier, and a gain inclination control unit which controls a gaininclination of the optical amplifier according to a gain inclinationdetected by the gain inclination detecting unit.

[0012] The optical amplifier apparatus according to another aspect ofthe present invention comprises an optical amplifier which amplifies aninput wavelength-multiplexed signal light, an optical variableattenuator which attenuates an output signal light of the opticalamplifier, an output detecting unit which detects an output level at anoutput side of the optical variable attenuator, an output control unitwhich controls the attenuation of an output signal light attenuated bythe optical variable attenuator according to an output level detected bythe output detecting unit, a gain inclination detecting unit whichdetects a gain inclination relating to a wavelength of the opticalamplifier, and a gain inclination control unit which controls a gaininclination by adjusting an output light of an excitation light sourceof the optical amplifier according to a gain inclination detected by thegain inclination detecting unit.

[0013] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, an optical variableattenuator which attenuates an output signal light of the opticalamplifier, an output detecting unit which detects an output level at anoutput side of the optical variable attenuator, an output control unitwhich controls an output light of an excitation light source of theoptical amplifier according to an output level detected by the outputdetecting unit, a gain inclination detecting unit which detects a gaininclination relating to a wavelength of the optical amplifier, and again inclination control unit which controls a gain inclination byadjusting the attenuation of an output signal light attenuated by theoptical variable attenuator according to a gain inclination detected bythe gain inclination detecting unit.

[0014] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of an input signal light to theoptical amplifier, a wavelength selecting unit which interrupts acompensation light at an output side of the optical amplifier, andtransmits only a signal light, an output detecting unit which detects anoutput level of a signal light at an output side of the wavelengthselecting unit, an output control unit which controls an output light ofthe compensation light source according to an output level detected bythe output detecting unit, a gain inclination detecting unit whichdetects a gain inclination of the optical amplifier, and a gaininclination control unit which controls a gain inclination by adjustingan output light of an excitation light source of the optical amplifieraccording to a gain inclination detected by the gain inclinationdetecting unit.

[0015] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, a wavelength selecting unit which interrupts acompensation light at an output side of the optical amplifier, andtransmits only a signal light, an output detecting unit which detects anoutput level of a signal light at an output side of the wavelengthselecting unit, an output control unit which controls an output light ofan excitation light source of the optical amplifier according to anoutput level detected by the output detecting unit, a gain inclinationdetecting unit which detects a gain inclination of the opticalamplifier, and a gain inclination control unit which controls a gaininclination by adjusting an output light of the compensation lightsource according to a gain inclination detected by the gain inclinationdetecting unit.

[0016] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, an optical variable attenuator which attenuates anoutput signal light of the optical amplifier, a wavelength selectingunit which interrupts a compensation light at an output side of theoptical variable attenuator, and transmits only a signal light, anoutput detecting unit which detects an output level of a signal light atan output side of the wavelength selecting unit, an output control unitwhich controls the attenuation of an output signal light of the opticalvariable attenuator according to an output level detected by the outputdetecting unit, a gain inclination detecting unit which detects a gaininclination of the optical amplifier, and a gain inclination controlunit which controls a gain inclination by adjusting an output light ofthe compensation light source according to a gain inclination detectedby the gain inclination detecting unit.

[0017] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, an optical variable attenuator which attenuates anoutput signal light of the optical amplifier, a wavelength selectingunit which interrupts a compensation light at an output side of theoptical variable attenuator, and transmits only a signal light, anoutput detecting unit which detects an output level of a signal light atan output side of the wavelength selecting unit, an output control unitwhich controls an output light of the compensation light sourceaccording to an output level detected by the output detecting unit, again inclination detecting unit which detects a gain inclination of theoptical amplifier, and a gain inclination control unit which controls again inclination by adjusting the attenuation of an output signal lightattenuated by the optical variable attenuator according to a gaininclination detected by the gain inclination detecting unit.

[0018] The optical amplifier apparatus according to still another aspectof the present invention comprises a first optical amplifier whichamplifies an input wavelength-multiplexed signal light, an opticalvariable attenuator which attenuates an output signal light of the firstoptical amplifier, a second optical amplifier which amplifies an outputsignal light of the optical variable attenuator, an output detectingunit which detects an output level at an output side of the secondoptical amplifier, an output control unit which controls the attenuationof an output signal light attenuated by the optical variable attenuatoraccording to an output level detected by the output detecting unit, again inclination detecting unit which detects a gain inclination of thesecond optical amplifier, and a gain inclination control unit whichcontrols a gain inclination by adjusting output lights of excitationlight sources of the first optical amplifier and the second opticalamplifier according to a gain inclination detected by the gaininclination detecting unit.

[0019] The optical amplifier apparatus according to still another aspectof the present invention comprises a first optical amplifier whichamplifies an input wavelength-multiplexed signal light, an opticalvariable attenuator which attenuates an output signal light of the firstoptical amplifier, a second optical amplifier which amplifies an outputsignal light of the optical variable attenuator, an output detectingunit which detects an output level at an output side of the secondoptical amplifier, an output control unit which controls output lightsof excitation light sources of the first optical amplifier and thesecond optical amplifier according to an output level detected by theoutput detecting unit, a gain inclination detecting unit which detectsagain inclination of the second optical amplifier, and a gaininclination control unit which controls a gain inclination by adjustingthe attenuation of an output signal light attenuated by the opticalvariable attenuator according to a gain inclination detected by the gaininclination detecting unit.

[0020] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, a first optical amplifier which amplifies the signallight, an optical variable attenuator which attenuates an output signallight of the first optical amplifier, a second optical amplifier whichamplifies an output signal light of the optical variable attenuator, awavelength selecting unit which interrupts a compensation light at anoutput side of the second optical amplifier, and transmits only a signallight, an output detecting unit which detects an output level of asignal light at an output side of the wavelength selecting unit, anoutput control unit which controls the attenuation of an output signallight attenuated by the optical variable attenuator according to anoutput level detected by the output detecting unit, a gain inclinationdetecting unit which detects a gain inclination of the second opticalamplifier, and a gain inclination control unit which controls a gaininclination by adjusting an output light of the compensation lightsource according to a gain inclination detected by the gain inclinationdetecting unit.

[0021] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, a first optical amplifier which amplifies the signallight, an optical variable attenuator which attenuates an output signallight of the first optical amplifier, a second optical amplifier whichamplifies an output signal light of the optical variable attenuator, awavelength selecting unit which interrupts a compensation light at anoutput side of the second optical amplifier, and transmits only a signallight, an output detecting unit which detects an output level of asignal light at an output side of the wavelength selecting unit, anoutput control unit which controls an output light of the compensationlight source according to an output level detected by the outputdetecting unit, a gain inclination detecting unit which detects a gaininclination of the second optical amplifier, and a gain inclinationcontrol unit which controls a gain inclination by adjusting theattenuation of an output signal light attenuated by the optical variableattenuator according to a gain inclination detected by the gaininclination detecting unit.

[0022] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light, a compensation lightsource which injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to theoptical amplifier, a first optical amplifier which amplifies the signallight, a second optical amplifier which amplifies an output signal lightof the first optical amplifier, a wavelength selecting unit whichinterrupts a compensation light at an output side of the second opticalamplifier, and transmits only a signal light, an output detecting unitwhich detects an output level of a signal light at an output side of thewavelength selecting unit, an output control unit which controls outputlights of excitation light sources of the first optical amplifier andthe second optical amplifier according to an output level detected bythe output detecting unit, a gain inclination detecting unit whichdetects a gain inclination of the optical amplifier, and a gaininclination control unit which controls a gain inclination by adjustingan output light of the compensation light source according to a gaininclination detected by the gain inclination detecting unit.

[0023] The optical amplifier apparatus according to still another aspectof the present invention comprises an optical amplifier which amplifiesan input wavelength-multiplexed signal light in an excitation lightsource, a compensation light source which injects a compensation lightthat propagates in a forward direction of a propagation direction of theinput signal light to the optical amplifier, a first optical amplifierwhich amplifies the signal light, a second optical amplifier whichamplifies an output signal light of the first optical amplifier, awavelength selecting unit which interrupts a compensation light at anoutput side of the second optical amplifier, and transmits only a signallight, an output detecting unit which detects an output level of asignal light at an output side of the wavelength selecting unit, anoutput control unit which controls an output light of the compensationlight source according to an output level detected by the outputdetecting unit, a gain inclination detecting unit which detects a gaininclination of the second optical amplifier, and a gain inclinationcontrol unit which controls a gain inclination by adjusting outputlights of an excitation light source of the first optical amplifier andan excitation light source of the second optical amplifier according toa gain inclination detected by the gain inclination detecting unit.

[0024] In the above-mentioned optical amplifier apparatus, the gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.

[0025] In the above-mentioned optical amplifier apparatus, the gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels ofthree or more waves.

[0026] In the above-mentioned optical amplifier apparatus, the opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and the gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.

[0027] In the above-mentioned optical amplifier apparatus, the opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and the gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a diagram showing a structure of a first embodiment ofan optical amplifier apparatus according to the present invention;

[0029]FIG. 2 is a graph showing an example of a gain inclination controlof the optical amplifier apparatus according to the first embodiment;

[0030]FIG. 3 is a diagram showing a structure of one example of a gaininclination detecting unit in an optical amplifier apparatus accordingto the present invention;

[0031]FIG. 4 is a diagram showing a structure of another example of again inclination detecting unit in an optical amplifier apparatusaccording to the present invention;

[0032]FIG. 5 is a diagram showing a structure of a second embodiment ofan optical amplifier apparatus according to the present invention;

[0033]FIG. 6 is a diagram showing a structure of a third embodiment ofan optical amplifier apparatus according to the present invention;

[0034]FIG. 7 is a diagram showing a structure of a fourth embodiment ofan optical amplifier apparatus according to the present invention;

[0035]FIG. 8 is a diagram showing a structure of a fifth embodiment ofan optical amplifier apparatus according to the present invention;

[0036]FIG. 9 is a diagram showing a structure of a sixth embodiment ofan optical amplifier apparatus according to the present invention;

[0037]FIG. 10 is a diagram showing a structure of a seventh embodimentof an optical amplifier apparatus according to the present invention;

[0038]FIG. 11 is a graph showing an example of a gain inclinationcontrol of the optical amplifier apparatus according to the seventhembodiment;

[0039]FIG. 12 is a diagram showing a structure of an eighth embodimentof an optical amplifier apparatus according to the present invention;

[0040]FIG. 13 is a diagram showing a structure of a ninth embodiment ofan optical amplifier apparatus according to the present invention;

[0041]FIG. 14 is a diagram showing a structure of a tenth embodiment ofan optical amplifier apparatus according to the present invention;

[0042]FIG. 15 is a diagram showing a structure of an eleventh embodimentof an optical amplifier apparatus according to the present invention;

[0043]FIG. 16 is a diagram showing a structure of a twelfth embodimentof an optical amplifier apparatus according to the present invention;and

[0044]FIG. 17 is a diagram showing an example of a conventionalstructure of an optical amplifier apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

[0045] Embodiments of an optical amplifier apparatus according to thepresent invention will be explained in detail below with reference tothe attached drawings.

[0046] First Embodiment:

[0047]FIG. 1 shows a first embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 1, a reference numeral 1denotes an input terminal, a reference numeral 2 denotes an outputterminal, a reference numeral 3 denotes an optical amplifier, areference numeral 4 denotes an optical variable attenuator, preferencenumeral 5 denotes an output detecting unit, a reference numeral 6denotes an output control unit, a reference numeral 7 denotes a gaininclination detecting unit, and a reference numeral 8 denotes a gaininclination control unit.

[0048] The optical amplifier apparatus of the first embodiment has theoptical amplifier 3, the optical variable attenuator 4, the outputdetecting unit 5, the output control unit 6, the gain inclinationdetecting unit 7, and the gain inclination control unit 8. The opticalamplifier 3 amplifies a wavelength-multipiexed signal light that isinput to the input terminal 1 connected with a not shown optical fiber.

[0049] The optical variable attenuator 4 variably attenuates an outputsignal light of the optical amplifier 3. Such an optical variableattenuator can be realized by, for example, a Faraday rotor thatutilizes a magnetooptic effect. It is also possible to structure thisoptical variable attenuator 4 with a device utilizing an electroopticeffect of LiNbO₃ or an acoustooptica effect.

[0050] The output detecting unit 5 detects an output level of theoptical variable attenuator 4, and outputs the detected value of theoutput level to the output control unit 6. The output control unit 6controls the attenuation of an output signal light attenuated by theoptical variable attenuator 4 according to the detection value, so thatan output level detected by the output detecting unit 5 becomesconstant.

[0051] The gain inclination detecting unit 7 detects a gain inclinationfrom an output of the optical variable attenuator 4, and outputs thisdetection value to the gain inclination control unit 8. The gaininclination control unit 8 adjusts an excitation light of the opticalamplifier 3 according to a gain inclination (a detection value) that isdetected by the gain inclination detecting unit 7.

[0052]FIG. 2 shows an example of a gain inclination control. In thisgain inclination control, the control is carried out by setting an inputlevel of the optical amplifier 3 to −8 dBm, an output level of theoptical variable attenuator 4 to +1 dBm, and a gain G of the opticalamplifier 3 to 9, 11, and 13 dB respectively. It can be understood fromthis that the gain inclination is controlled by controlling only thegain G of the optical amplifier 3. Based on the above control, it ispossible to realize an optical amplifier apparatus capable ofcontrolling the gain inclination as well as making constant the opticaloutput level, in the amplification of a wavelength-multiplexed signallight.

[0053] A detailed example of the gain inclination detecting unit 7 willbe explained next with reference to FIG. 3. This gain inclinationdetecting unit 7 is for branching a wavelength-multiplexed signal light,and detecting a gain inclination from optical signal levels of ashortest wavelength λ1 and a longest wavelength λn. The wavelengthselecting unit 9 divides only the components of the shortest wavelengthλ1 and the longest wavelength λn out of a wavelength-multiplexed signallight that is output from the optical amplifier 3, more strictly, awavelength-multiplexed signal light output of the optical variableattenuator 4. The wavelength selecting unit 9 then sends the respectivecomponents to optical detecting units 10-1 and 10-n. It is possible tostructure the wavelength selecting unit 9 with ease by using an opticalfilter. It is possible to use a photodiode, an avalanche photodiode, anda photocounter for the optical detecting units 10-1 and 10-nrespectively.

[0054] The shortest wavelength λ1 is sent to the optical detecting unit10-1, and the longest wavelength λn is sent to the optical detectingunit 10-n. The optical detecting units 10-1 and 10-n convert thereceived lights into electric signals respectively, and detect a levelof the shortest wavelength λ1 and a level of the longest wavelength λnof the signal light respectively. The detection values are sent to again inclination detecting circuit 11. The gain inclination detectingcircuit 11 can detect a gain inclination by comparing the level of theshortest wavelength λ1 with the level of the longest wavelength λn.

[0055] In this first embodiment, it is explained that the gaininclination is detected from the components of the shortest wavelengthλ1 and the level of the longest wavelength λn. However, it is alsopossible to detect a gain inclination from optical signal levels ofthree or more waves. In this case, wavelength selecting units may beinserted by a number of multiplexed wavelengths of awavelength-multiplexed optical signal. These wavelength selecting unitsdivide the signal light having the wavelengthsλ1, λ2, . . . , and λnmultiplexed together, and send wavelength signals of these wavelengthsλ1, λ2, . . . , and λn to respective optical detecting units.Thereafter, the operation similar to that of the above embodiment iscarried out.

[0056] It is also possible to arrange such that a transmitter sidesuperimposes mutually different frequency components on the opticalsignals of the shortest wavelength λ1 and the longest wavelength λ2respectively. Then, the frequency components superimposed on the opticalsignals of respective wavelengths may be detected to detect a gaininclination. When a low-frequency tone signal superimposing system isemployed, it is possible to accurately control a gain without aninfluence of cumulative natural discharge optical noise. An example of astructure this gain inclination detecting unit 7 will be explained withreference to FIG. 4. The gain inclination detecting unit 7 isconstructed of an optical detecting unit 10, band-pass filters (BPF's)12-1 and 12-n, level detecting units 13-1 and 13-n, and a gaininclination detecting circuit 14.

[0057] A wavelength-multiplexed signal light output of an opticalrepeater is input to the optical detecting unit 10. The band-passfilters 12-1 and 12-n extract respectively the frequency componentsallocated to wavelengths, from a detection signal. The level detectingunits 13-1 and 13-n detect respectively the levels of signal lights ofthese frequency components.

[0058] In this case, the band-pass filter 12-1 extracts a frequencycomponent f1 that has been superimposed on the shortest wavelength λ1,and the band-pass filter 12-n extracts a frequency component fn that hasbeen superimposed on the longest wavelength λn. The level detectingunits 13-1 and 13-n detect respectively the levels of the extractedfrequency components f1 and fn, and send the respective detection valuesto the gain inclination detecting circuit 14. The gain inclinationdetecting circuit 14 detects a gain inclination by comparing the levelsof these frequency components.

[0059] In this first embodiment, it is explained that the gaininclination is detected by superimposing mutually different frequencycomponents on only the shortest wavelength and the longest wavelength.However, it is also possible to superimpose mutually different frequencycomponents on optical signals of three or more waves. The gaininclination detecting circuit detects the superimposed frequencycomponents thereby to detect a gain inclination. In this case, it isnecessary to mutually differentiate the wavelengths of the lights thatare output from respective optical transmitters, and mutuallydifferentiate the frequencies of sinusoidal wave signals that aresuperimposed. Further, band-pass filters and level detectorscorresponding to the number of superimposed frequencies need to beprepared respectively for the gain inclination detecting unit 7. Thecenter frequency of each band-pass filter is corresponded to thefrequency of each sinusoidal wave signal that is superimposed.

[0060] Second Embodiment:

[0061]FIG. 5 shows a second embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 5, portions corresponding tothose in FIG. 1 are attached with like reference numerals of theportions in FIG. 1, and their explanation will be omitted.

[0062] The optical amplifier apparatus of the second embodiment has theoptical amplifier 3, the optical variable attenuator 4, the outputdetecting unit 5, an output control unit 15, the gain inclinationdetecting unit 7, and a gain inclination control unit 16.

[0063] The output control unit 15 controls an output light of anexcitation light source of the optical amplifier 3 so that an outputlevel becomes constant, according to an output level detected by theoutput detecting unit 5. The gain inclination control unit 16 controlsagain inclination to an optimum value by adjusting the attenuation of anoutput signal light attenuated by the optical variable attenuator 4according to a gain inclination detected by the gain inclinationdetecting unit 7.

[0064] Based on the above control, in this second embodiment also, likein the first embodiment, it is possible to realize an optical amplifierapparatus capable of controlling the gain inclination as well as makingconstant the optical output level, in the amplification of awavelength-multiplexed signal light.

[0065] Third Embodiment:

[0066]FIG. 6 shows a third embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 6, portions corresponding tothose in FIG. 1 are attached with like reference numerals of theportions in FIG. 1, and their explanation will be omitted.

[0067] The optical amplifier apparatus of the third embodiment has theoptical amplifier 3, a compensation light source 17, a wavelengthselecting unit 18, the output detecting unit 5, an output control unit19, the gain inclination detecting unit 7, and the gain inclinationcontrol unit 8.

[0068] The compensation light source 17 injects to an input side of theoptical amplifier 3 a compensation light that propagates in a forwarddirection of a propagation direction of an input signal light to theoptical amplifier 3. In this case, the gain of the optical amplifier 3changes depending on total power of a signal light and a compensationlight. Therefore, the power of the signal light changes depending on thepower of the compensation light, and then the level of awavelength-multiplexed signal light at an output terminal 2 changes. Thewavelength selecting unit 18 is constructed of an optical rejectionfilter or a coupler, for example. This wavelength selecting unit 18interrupts only a light of the wavelength of the compensation lightsource 17 (the compensation light) out of the output light of theoptical amplifier 3, and transmits other components of thewavelength-multiplexed signal light.

[0069] The output detecting unit 5 detects an output level of awavelength-multiplexed signal light at an output side of the wavelengthselecting unit 18, and outputs a detection value to the output controlunit 19. The output control unit 19 controls an output level of thecompensation light source 17 according to the detection value so thatthe output level detected by the output detecting unit 5 becomesconstant. The gain inclination detecting unit 7 detects a gaininclination from an output of the optical variable attenuator 4, andoutputs a detection value to the gain inclination control unit 8. Thegain inclination control unit 8 controls the gain inclination to anoptimum value by adjusting an excitation light of the optical amplifier3 according to a gain inclination (a detection value) detected by thegain inclination detecting unit 7.

[0070] Therefore, in this third embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0071] Fourth Embodiment:

[0072]FIG. 7 shows a fourth embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 7, portions corresponding tothose in FIG. 5 and FIG. 6 are attached with like reference numerals ofthe portions in FIG. 5 and FIG. 6, and their explanation will beomitted.

[0073] The optical amplifier apparatus of the fourth embodiment has theoptical amplifier 3, the compensation light source 17, the wavelengthselecting unit 18, the output detecting unit 5, the output control unit15, the gain inclination detecting unit 7, and a gain inclinationcontrol unit 20.

[0074] The output detecting unit 5 detects an output level of awavelength-multiplexed signal light at an output side of the wavelengthselecting unit 18, and outputs a detection value to the output controlunit 15. The output control unit 15 controls an output light of anexcitation light source of the optical amplifier 3 according to thedetection value so that the output level detected by the outputdetecting unit 5 becomes constant. The gain inclination detecting unit 7detects a gain inclination from an output of the optical variableattenuator 4, and outputs a detection value to the gain inclinationcontrol unit 20. The gain inclination control unit 20 controls the gaininclination to an optimum value by adjusting an output level of thecompensation light source 17 according to a gain inclination (i.e. avalue of the gain inclination) detected by the gain inclinationdetecting unit 7.

[0075] Therefore, in this fourth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0076] Fifth Embodiment:

[0077]FIG. 8 shows a fifth embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 8, portions corresponding tothose in FIG. 1 and FIG. 7 are attached with like reference numerals ofthe portions in FIG. 1 and FIG. 7, and their explanation will beomitted.

[0078] The optical amplifier apparatus of the fifth embodiment has theoptical amplifier 3, the optical variable attenuator 4, the compensationlight source 17, the wavelength selecting unit 18, the output detectingunit 5, the output control unit 6, the gain inclination detecting unit7, and the gain inclination control unit 20.

[0079] The output detecting unit 5 detects an output level of awavelength-multiplexed signal light at an output side of the wavelengthselecting unit 18, and outputs a detection value to the output controlunit 6. The output control unit 6 controls the attenuation of an outputsignal light of the optical variable attenuator 4 according to thedetection value so that the output level detected by the outputdetecting unit 5 becomes constant. The gain inclination detecting unit 7detects a gain inclination from an output of the optical variableattenuator 4, and outputs a detection value to the gain inclinationcontrol unit 20. The gain inclination control unit 20 controls the gaininclination to an optimum value by adjusting an output level of thecompensation light source 17 according to a gain inclination (i.e. avalue of the gain inclination) detected by the gain inclinationdetecting unit 7, in a similar manner to that of the fourth embodiment.

[0080] Therefore, in this fifth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0081] Sixth Embodiment:

[0082]FIG. 9 shows a sixth embodiment of an optical amplifier apparatusaccording to the present invention. In FIG. 9, portions corresponding tothose in FIG. 5 and FIG. 6 are attached with like reference numerals ofthe portions in FIG. 5 and FIG. 6, and their explanation will beomitted.

[0083] The optical amplifier apparatus of the sixth embodiment has theoptical amplifier 3, the optical variable attenuator 4, the compensationlight source 17, the wavelength selecting unit 18, the output detectingunit 5, the output control unit 19, the gain inclination detecting unit7, and the gain inclination control unit 16.

[0084] The output detecting unit 5 detects an output level of awavelength-multiplexed signal light at an output side of the wavelengthselecting unit 18, and outputs a detection value to the output controlunit 19. The output control unit 19 controls an output level of thecompensation light source 17 according to the detection value so thatthe output level detected by the output detecting unit 5 becomesconstant, in a similar manner to that of the third embodiment. The gaininclination detecting unit 7 detects a gain inclination from an outputof the optical variable attenuator 4, and outputs a detection value tothe gain inclination control unit 16. The gain inclination control unit16 controls the gain inclination to an optimum value by adjusting theattenuation of an output signal light attenuated by the optical variableattenuator 4 according to a gain inclination detected by the gaininclination detecting unit 7, in a similar manner to that of the secondembodiment.

[0085] Therefore, in this sixth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0086] Seventh Embodiment:

[0087]FIG. 10 shows a seventh embodiment of an optical amplifierapparatus according to the present invention. In FIG. 10, portionscorresponding to those in FIG. 1 are attached with like referencenumerals of the portions in FIG. 1, and their explanation will beomitted.

[0088] The optical amplifier apparatus of the seventh embodiment has thefirst and second optical amplifiers 3-1 and 3-2, the optical variableattenuator 4, output detecting units 5-1 and 5-2, the output controlunit 6, the gain inclination detecting unit 7, a gain inclinationcontrol unit 23, input detecting units 21-l and 21-2, and gain controlunits 24-1 and 24-2.

[0089] The gain of the first optical amplifier 3-1 and the gain of thesecond optical amplifier 3-2 are always the same. The output detectingunit 5-2 monitors an output level of the second optical amplifier 3-2,and the output control unit 6 control the optical variable attenuator 4so that this output level becomes constant. Attenuation A of the opticalvariable attenuator 4 is uniquely determined as A=P2−P1−2×G, where Grepresents a gain of the first optical amplifier 3-1 as well as again ofthe second optical amplifier 3-2, P−1 represents an input level of thefirst optical amplifier 3-1, and P2 represents an output level of thesecond optical amplifier 3-2. On the other hand, a gain inclination D isexpressed as a function of the gain G, like D=D (G).

[0090] In other words, it is possible to uniquely determine the gaininclination D based on the gain G of the first optical amplifier 3-1 andthe second optical amplifier 3-2. It is possible to control the gaininclination by giving only the gain G of the optical amplifier from theoutside, without changing the input and output levels.

[0091]FIG. 11 shows an example of a gain inclination control. In thisexample of the gain inclination control, the control is carried out bysetting an input level P1 of the first optical amplifier 3-1 to −8 dBm,an output level P2 of the second optical amplifier 3-2 to +3 dBm, andthe gain G of the optical amplifiers to 9, 11, and 13 dB respectively.It can be understood that the gain inclination is controlled bycontrolling only the gain G of the optical amplifiers.

[0092] Therefore, in this seventh embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0093] Eighth Embodiment:

[0094]FIG. 12 shows an eighth embodiment of an optical amplifierapparatus according to the present invention. In FIG. 12, portionscorresponding to those in FIG. 5 and FIG. 10 are attached with likereference numerals of the portions in FIG. 5 and FIG. 10, and theirexplanation will be omitted.

[0095] The optical amplifier apparatus of the eighth embodiment has thefirst and second optical amplifiers 3-1 and 3-2, the optical variableattenuator 4, output detecting units 5-1 and 5-2, the output controlunit 15, the gain inclination detecting unit 7, the gain inclinationcontrol unit 16, the input detecting units 21-1 and 21-2, and the gaincontrol units 24-1 and 24-2.

[0096] The output control unit 15 controls output lights of excitationlight sources of the first optical amplifier 3-1 and the second opticalamplifier 3-2 so that an output level becomes constant according to anoutput level detected by the output detecting unit 5-2. In this case,the gain control unit 24-1 and the gain control unit 24-2 control theoutput lights of the excitation light sources so that the gain of thefirst optical amplifier 3-1 becomes equal to the gain of the secondoptical amplifier 3-2. The gain inclination control unit 16 controls thegain inclination to an optimum value by adjusting the attenuation of anoutput signal light attenuated by the optical variable attenuator 4according to a gain inclination detected by the gain inclinationdetecting unit 7.

[0097] Therefore, in this eighth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0098] Ninth Embodiment:

[0099]FIG. 13 shows a ninth embodiment of an optical amplifier apparatusaccording to the present invention.

[0100] The optical amplifier apparatus of the ninth embodiment has thefirst and second optical amplifiers 3-1 and 3-2, the optical variableattenuator 4, a first compensation light source 17-1, a secondcompensation light source 17-2, a first wavelength selecting unit 18-1,a second wavelength selecting unit 18-2, output detecting units 5-1 and5-2, the output control unit 6, the gain inclination detecting unit 7,the gain inclination control unit 20, the input detecting units 21-1 and21-2, and the gain control units 24-1 and 24-2. The reference number 1denotes the input terminal, and 2 denotes the output terminal.

[0101] The output detecting unit 5-2 detects an output level of awavelength-multiplexed signal light at an output side of the secondwavelength selecting unit 18-2, and outputs a detection value to theoutput control unit 6. The output control unit 6 controls theattenuation of an output signal light attenuated by the optical variableattenuator 4 according to the detection value so that the output leveldetected by the output detecting unit 5-2 becomes constant. The gaininclination detecting unit 7 detects a gain inclination from an outputof the second wavelength selecting unit 18-2, and outputs a detectionvalue to the gain inclination control unit 20. The gain inclinationcontrol unit 20 controls the gain inclination to an optimum value byadjusting output levels of the first compensation light source 17-1 andthe second compensation light source 17-2 according to a gaininclination (a detection value) detected by the gain inclinationdetecting unit 7.

[0102] Therefore, in this ninth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0103] Tenth Embodiment:

[0104] FIG.14 shows a tenth embodiment of an optical amplifier apparatusaccording to the present invention.

[0105] The optical amplifier apparatus of the tenth embodiment has thefirst and second optical amplifiers 3-1 and 3-2, the optical variableattenuator 4, the first and second compensation light sources 17-1 and17-2, the first and second wavelength selecting units 18-1 and 18-2, theoutput detecting units 5-1 and 5-2, the output control unit 19, the gaininclination detecting unit 7, the gain inclination control unit 16, theinput detecting units 21-1 and 21-2, and the gain control units 24-1 and24-2. The reference number 1 denotes the input terminal, and 2 denotesthe output terminal.

[0106] The output detecting unit 5-2 detects an output level of awavelength-multiplexed signal light at an output side of the secondwavelength selecting unit 18-2, and outputs a detection value to theoutput control unit 19. The output control unit 19 controls outputlevels of the first compensation light source 17-1 and the secondcompensation light source 17-2 according to the detection value so thatthe output level detected by the output detecting unit 5-2 becomesconstant. The gain inclination detecting unit 7 detects a gaininclination from an output of the second wavelength selecting unit 18-2,and outputs a detection value to the gain inclination control unit 16.The gain inclination control unit 16 controls the gain inclination to anoptimum value by adjusting the attenuation of an output signal lightattenuated by the optical variable attenuator 4 according to a gaininclination (a detection value) detected by the gain inclinationdetecting unit 7.

[0107] Therefore, in this tenth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0108] Eleventh Embodiment:

[0109]FIG. 15 shows an eleventh embodiment of an optical amplifierapparatus according to the present invention.

[0110] The optical amplifier apparatus of the eleventh embodiment hasthe first and second optical amplifiers 3-1 and 3-2, the first andsecond compensation light sources 17-1 and 17-2, the first and secondwavelength selecting units 18-1 and 18-2, the output detecting units 5-1and 5-2, the output control unit 19, the gain inclination detecting unit7, the gain inclination control unit 23, the input detecting units 21-1and 21-2, and the gain control units 24-1 and 24-2. The reference number1 denotes the input terminal, and 2 denotes the output terminal.

[0111] The output detecting unit 5-2 detects an output level of awavelength-multiplexed signal light at an output side of the secondwavelength selecting unit 18-2, and outputs a detection value to theoutput control unit 19. The output control unit 19 controls outputlevels of the first compensation light source 17-1 and the secondcompensation light source 17-2 according to the detection value so thatthe output level detected by the output detecting unit 5-2 becomesconstant. The gain inclination detecting unit 7 detects a gaininclination from an output of the second wavelength selecting unit 18-2,and outputs a detection value to the gain inclination control unit 23.The gain inclination control unit 23 controls the gain inclination to anoptimum value by adjusting output lights of excitation light sources ofthe first optical amplifier 3-1 and the second optical amplifier 3-2according to a gain inclination (a detection value) detected by the gaininclination detecting unit 7.

[0112] Therefore, in this eleventh embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0113] Twelfth Embodiment:

[0114]FIG. 16 shows a twelfth embodiment of an optical amplifierapparatus according to the present invention.

[0115] The optical amplifier apparatus of the twelfth embodiment has thefirst and second optical amplifiers 3-1 and 3-2, the first and secondcompensation light sources 17-1 and 17-2, the first and secondwavelength selecting units 18-1 and 18-2, the output detecting units 5-1and 5-2, the output control unit 15, the gain inclination detecting unit7, the gain inclination control unit 20, the input detecting units 21-1and 21-2, and the gain control units 24-1 and 24-2. The reference number1 denotes the input terminal, and 2 denotes the output terminal.

[0116] The output detecting unit 5-2 detects an output level of awavelength-multiplexed signal light at an output side of the secondwavelength selecting unit 18-2, and outputs a detection value to theoutput control unit 15. The output control unit 15 controls outputlights of excitation light sources of the first optical amplifier 3-1and the second optical amplifier 3-2 according to the detection value sothat the output level detected by the output detecting unit 5-2 becomesconstant. The gain inclination detecting unit 7 detects a gaininclination from an output of the second wavelength selecting unit 18-2,and outputs a detection value to the gain inclination control unit 20.The gain inclination control unit 20 controls the gain inclination to anoptimum value by adjusting output lights of the first compensation lightsource 17-1 and the second compensation light source 17-2 according to again inclination (a detection value) detected by the gain inclinationdetecting unit 7.

[0117] Therefore, in this twelfth embodiment also, like in the firstembodiment, it is possible to realize an optical amplifier apparatuscapable of controlling the gain inclination as well as making constantthe optical output level, in the amplification of awavelength-multiplexed signal light.

[0118] As can be understood from the above explanation, according to oneaspect of the present invention, the output control unit controls anoutput level of the optical amplifier according to an output leveldetected by the output detecting unit. The gain inclination control unitcontrols a gain inclination of the optical amplifier according to a gaininclination detected by the gain inclination detecting unit. Therefore,it is possible to realize an optical amplifier apparatus capable ofcontrolling the gain inclination as well as making constant the opticaloutput level, in the amplification of a wavelength-multiplexed signallight.

[0119] According to another aspect of the invention, the output controlunit controls the attenuation of an output signal light attenuated bythe optical variable attenuator according to an output level detected bythe output detecting unit. The gain inclination control unit controls again inclination by adjusting an output light of an excitation lightsource of the optical amplifier according to a gain inclination detectedby the gain inclination detecting unit. Therefore, it is possible torealize an optical amplifier apparatus capable of controlling the gaininclination as well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0120] According to still another aspect of the invention, the outputcontrol unit controls an output light of an excitation light source ofthe optical amplifier according to an output level detected by theoutput detecting unit. The gain inclination control unit controls againinclination by adjusting the attenuation of an output signal lightattenuated by the optical variable attenuator according to a gaininclination detected by the gain inclination detecting unit. Therefore,it is possible to realize an optical amplifier apparatus capable ofcontrolling the gain inclination as well as making constant the opticaloutput level, in the amplification of a wavelength-multiplexed signallight.

[0121] According to still another aspect of the invention, the outputcontrol unit controls an output light of the compensation light sourceaccording to an output level detected by the output detecting unit. Thegain inclination control unit adjusts an output light of an excitationlight source of the optical amplifier according to a gain inclinationdetected by the gain inclination detecting unit. Therefore, it ispossible to realize an optical amplifier apparatus capable ofcontrolling the gain inclination as well as making constant the opticaloutput level, in the amplification of a wavelength-multiplexed signallight.

[0122] According to still another aspect of the invention, the outputcontrol unit controls an output light of an excitation light source ofthe optical amplifier according to an output level detected by theoutput detecting unit. The gain inclination adjusts an output light ofthe compensation light source according to a gain inclination detectedby the gain inclination detecting unit. Therefore, it is possible torealize an optical amplifier apparatus capable of controlling the gaininclination as well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0123] According to still another aspect of the invention, the outputcontrol unit controls the attenuation of an output signal light of theoptical variable attenuator according to an output level detected by theoutput detecting unit. The gain inclination control unit adjusts anoutput light of the compensation light source according to a gaininclination detected by the gain inclination detecting unit. Therefore,it is possible to realize an optical amplifier apparatus capable ofcontrolling the gain inclination as well as making constant the opticaloutput level, in the amplification of a wavelength-multiplexed signallight.

[0124] According to still another aspect of the invention, the outputcontrol unit controls an output light of the compensation light sourceaccording to an output level detected by the output detecting unit. Thegain inclination control unit controls a gain inclination by adjustingthe attenuation of an output signal light attenuated by the opticalvariable attenuator according to a gain inclination detected by the gaininclination detecting unit. Therefore, it is possible to realize anoptical amplifier apparatus capable of controlling the gain inclinationas well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0125] According to still another aspect of the invention, the outputcontrol unit controls the attenuation of an output signal lightattenuated by the optical variable attenuator according to an outputlevel detected by the output detecting unit. The gain inclinationcontrol unit controls a gain inclination by adjusting output lights ofexcitation light sources of the first optical amplifier and the secondoptical amplifier according to a gain inclination detected by the gaininclination detecting unit. Therefore, it is possible to realize anoptical amplifier apparatus capable of controlling the gain inclinationas well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0126] According to still another aspect of the invention, the outputcontrol unit controls output lights of excitation light sources of thefirst optical amplifier and the second optical amplifier according to anoutput level detected by the output detecting unit. The gain inclinationcontrol unit controls a gain inclination by adjusting the attenuation ofan output signal light attenuated by the optical variable attenuatoraccording to a gain inclination detected by the gain inclinationdetecting unit. Therefore, it is possible to realize an opticalamplifier apparatus capable of controlling the gain inclination as wellas making constant the optical output level, in the amplification of awavelength-multiplexed signal light.

[0127] According to still another aspect of the invention, the outputcontrol unit controls the attenuation of an output signal lightattenuated by the optical variable attenuator according to an outputlevel detected by the output detecting unit. The gain inclinationcontrol unit controls a gain inclination by adjusting an output light ofthe compensation light source according to a gain inclination detectedby the gain inclination detecting unit. Therefore, it is possible torealize an optical amplifier apparatus capable of controlling the gaininclination as well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0128] According to still another aspect of the invention, the outputcontrol unit controls an output light of the compensation light sourceaccording to an output level detected by the output detecting unit. Thegain inclination control unit controls a gain inclination by adjustingthe attenuation of an output signal light attenuated by the opticalvariable attenuator according to a gain inclination detected by the gaininclination detecting unit. Therefore, it is possible to realize anoptical amplifier apparatus capable of controlling the gain inclinationas well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0129] According to still another aspect of the invention, the outputcontrol unit controls output lights of excitation light sources of thefirst optical amplifier and the second optical amplifier according to anoutput of the output detecting unit. The gain inclination control unitcontrols a gain inclination by adjusting an output light of thecompensation light source according to a gain inclination detected bythe gain inclination detecting unit. Therefore, it is possible torealize an optical amplifier apparatus capable of controlling the gaininclination as well as making constant the optical output level, in theamplification of a wavelength-multiplexed signal light.

[0130] According to still another aspect of the invention, the outputcontrol unit controls an output light of the compensation light sourceaccording to an output level detected by the output detecting unit. Thegain inclination control unit controls a gain inclination by adjustingoutput lights of an excitation light source of the first opticalamplifier and an excitation light source of the second optical amplifieraccording to a gain inclination detected by the gain inclinationdetecting unit. Therefore, it is possible to realize an opticalamplifier apparatus capable of controlling the gain inclination as wellas making constant the optical output level, in the amplification of awavelength-multiplexed signal light.

[0131] Moreover, the gain inclination detecting unit branches awavelength-multiplexed signal light, and detects a gain inclination fromoptical signal levels of a shortest wave and a longest wave. Therefore,it is possible to properly detect the gain inclination.

[0132] Furthermore, the gain inclination detecting unit branches awavelength-multiplexed signal light, and detects a gain inclination fromoptical signal levels of three or more waves. Therefore, it is possibleto properly detect the gain inclination.

[0133] Moreover, the gain inclination detecting unit detects frequencycomponents superimposed on the optical signals of respective wavelengthsthereby to detect a gain inclination. Therefore, it is possible toproperly detect the gain inclination.

[0134] Furthermore, the gain inclination detecting unit detectsfrequency components superimposed on the optical signals thereby todetect a gain inclination. Therefore, it is possible to properly detectthe gain inclination.

INDUSTRIAL APPLICABILITY

[0135] As explained above, the optical amplifier apparatus according tothe present invention is suitable for application to an optical repeaterand the like for wavelength multiplex transmission.

1. An optical amplifier apparatus comprising: an optical amplifier whichamplifies an input signal light; an output detecting unit which detectsan output level of said optical amplifier; an output control unit whichcontrols an output level of said optical amplifier according to anoutput level detected by said output detecting unit; a gain inclinationdetecting unit which detects a gain inclination relating to a wavelengthof said optical amplifier; and a gain inclination control unit whichcontrols a gain inclination of said optical amplifier according to again inclination detected by said gain inclination detecting unit. 2.The optical amplifier apparatus according to claim 1, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.
 3. The optical amplifier apparatusaccording to claim 1, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of three or more waves.
 4. Theoptical amplifier apparatus according to claim 1, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.
 5. The optical amplifier apparatus according to claim 1,wherein said optical amplifier apparatus inputs a wavelength-multiplexedsignal light having mutually different frequency components superimposedon optical signals of three or more waves respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals thereby to detect a gain inclination.
 6. An opticalamplifier apparatus comprising: an optical amplifier which amplifies aninput wavelength-multiplexed signal light; an optical variableattenuator which attenuates an output signal light of said opticalamplifier; an output detecting unit which detects an output level at anoutput side of said optical variable attenuator; an output control unitwhich controls the attenuation of an output signal light attenuated bysaid optical variable attenuator according to an output level detectedby said output detecting unit; a gain inclination detecting unit whichdetects a gain inclination relating to a wavelength of said opticalamplifier; and a gain inclination control unit which controls a gaininclination by adjusting an output light of an excitation light sourceof said optical amplifier according to a gain inclination detected bysaid gain inclination detecting unit.
 7. The optical amplifier apparatusaccording to claim 6, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of a shortest wave and a longestwave.
 8. The optical amplifier apparatus according to claim 6, whereinsaid gain inclination detecting unit branches a wavelength-multiplexedsignal light, and detects a gain inclination from optical signal levelsof three or more waves.
 9. The optical amplifier apparatus according toclaim 6, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 10. Theoptical amplifier apparatus according to claim 6, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 11. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; an optical variable attenuatorwhich attenuates an output signal light of said optical amplifier; anoutput detecting unit which detects an output level at an output side ofsaid optical variable attenuator; an output control unit which controlsan output light of an excitation light source of said optical amplifieraccording to an output level detected by said output detecting unit; again inclination detecting unit which detects a gain inclinationrelating to a wavelength of said optical amplifier; and a gaininclination control unit which controls a gain inclination by adjustingthe attenuation of an output signal light attenuated by said opticalvariable attenuator according to a gain inclination detected by saidgain inclination detecting unit.
 12. The optical amplifier apparatusaccording to claim 11, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of a shortest wave and a longestwave.
 13. The optical amplifier apparatus according to claim 11, whereinsaid gain inclination detecting unit branches a wavelength-multiplexedsignal light, and detects a gain inclination from optical signal levelsof three or more waves.
 14. The optical amplifier apparatus according toclaim 11, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 15. Theoptical amplifier apparatus according to claim 11, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 16. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of an input signal light to said opticalamplifier; a wavelength selecting unit which interrupts a compensationlight at an output side of said optical amplifier, and transmits only asignal light; an output detecting unit which detects an output level ofa signal light at an output side of said wavelength selecting unit; anoutput control unit which controls an output light of said compensationlight source according to an output level detected by said outputdetecting unit; a gain inclination detecting unit which detects a gaininclination of said optical amplifier; and a gain inclination controlunit which controls a gain inclination by adjusting an output light ofan excitation light source of said optical amplifier according to a gaininclination detected by said gain inclination detecting unit.
 17. Theoptical amplifier apparatus according to claim 16, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.
 18. The optical amplifier apparatusaccording to claim 16, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of three or more waves.
 19. Theoptical amplifier apparatus according to claim 16, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.
 20. The optical amplifier apparatus according to claim 16,wherein said optical amplifier apparatus inputs a wavelength-multiplexedsignal light having mutually different frequency components superimposedon optical signals of three or more waves respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals thereby to detect a gain inclination.
 21. An opticalamplifier apparatus comprising: an optical amplifier which amplifies aninput wavelength-multiplexed signal light; a compensation light sourcewhich injects a compensation light that propagates in a forwarddirection of a propagation direction of the input signal light to saidoptical amplifier; a wavelength selecting unit which interrupts acompensation light at an output side of said optical amplifier, andtransmits only a signal light; an output detecting unit which detects anoutput level of a signal light at an output side of said wavelengthselecting unit; an output control unit which controls an output light ofan excitation light source of said optical amplifier according to anoutput level detected by said output detecting unit; a gain inclinationdetecting unit which detects a gain inclination of said opticalamplifier; and a gain inclination control unit which controls a gaininclination by adjusting an output light of said compensation lightsource according to a gain inclination detected by said gain inclinationdetecting unit.
 22. The optical amplifier apparatus according to claim21, wherein said gain inclination detecting unit branches awavelength-multiplexed signal light, and detects a gain inclination fromoptical signal levels of a shortest wave and a longest wave.
 23. Theoptical amplifier apparatus according to claim 21, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels ofthree or more waves.
 24. The optical amplifier apparatus according toclaim 21, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 25. Theoptical amplifier apparatus according to claim 21, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 26. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of the input signal light to said opticalamplifier; an optical variable attenuator which attenuates an outputsignal light of said optical amplifier; a wavelength selecting unitwhich interrupts a compensation light at an output side of said opticalvariable attenuator, and transmits only a signal light; an outputdetecting unit which detects an output level of a signal light at anoutput side of said wavelength selecting unit; an output control unitwhich controls the attenuation of an output signal light of said opticalvariable attenuator according to an output level detected by said outputdetecting unit; a gain inclination detecting unit which detects a gaininclination of said optical amplifier; and a gain inclination controlunit which controls a gain inclination by adjusting an output light ofsaid compensation light source according to a gain inclination detectedby said gain inclination detecting unit.
 27. The optical amplifierapparatus according to claim 26, wherein said gain inclination detectingunit branches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of a shortest wave and a longestwave.
 28. The optical amplifier apparatus according to claim 26, whereinsaid gain inclination detecting unit branches a wavelength-multiplexedsignal light, and detects a gain inclination from optical signal levelsof three or more waves.
 29. The optical amplifier apparatus according toclaim 26, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 30. Theoptical amplifier apparatus according to claim 26, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 31. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of the input signal light to said opticalamplifier; an optical variable attenuator which attenuates an outputsignal light of said optical amplifier; a wavelength selecting unitwhich interrupts a compensation light at an output side of said opticalvariable attenuator, and transmits only a signal light; an outputdetecting unit which detects an output level of a signal light at anoutput side of said wavelength selecting unit; an output control unitwhich controls an output light of said compensation light sourceaccording to an output level detected by said output detecting unit; again inclination detecting unit which detects a gain inclination of saidoptical amplifier; and a gain inclination control unit which controls again inclination by adjusting the attenuation of an output signal lightattenuated by said optical variable attenuator according to a gaininclination detected by said gain inclination detecting unit.
 32. Theoptical amplifier apparatus according to claim 31, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.
 33. The optical amplifier apparatusaccording to claim 31, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of three or more waves.
 34. Theoptical amplifier apparatus according to claim 31, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.
 35. The optical amplifier apparatus according to claim 31,wherein said optical amplifier apparatus inputs a wavelength-multiplexedsignal light having mutually different frequency components superimposedon optical signals of three or more waves respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals thereby to detect a gain inclination.
 36. An opticalamplifier apparatus comprising: a first optical amplifier whichamplifies an input wavelength-multiplexed signal light; an opticalvariable attenuator which attenuates an output signal light of saidfirst optical amplifier; a second optical amplifier which amplifies anoutput signal light of said optical variable attenuator; an outputdetecting unit which detects an output level at an output side of saidsecond optical amplifier; an output control unit which controls theattenuation of an output signal light attenuated by said opticalvariable attenuator according to an output level detected by said outputdetecting unit; a gain inclination detecting unit which detects a gaininclination of said second optical amplifier; and a gain inclinationcontrol unit which controls a gain inclination by adjusting outputlights of excitation light sources of said first optical amplifier andsaid second optical amplifier according to a gain inclination detectedby said gain inclination detecting unit.
 37. The optical amplifierapparatus according to claim 36, wherein said gain inclination detectingunit branches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of a shortest wave and a longestwave.
 38. The optical amplifier apparatus according to claim 36, whereinsaid gain inclination detecting unit branches a wavelength-multiplexedsignal light, and detects a gain inclination from optical signal levelsof three or more waves.
 39. The optical amplifier apparatus according toclaim 36, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 40. Theoptical amplifier apparatus according to claim 36, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 41. An optical amplifier apparatuscomprising: a first optical amplifier which amplifies an inputwavelength-multiplexed signal light; an optical variable attenuatorwhich attenuates an output signal light of said first optical amplifier;a second optical amplifier which amplifies an output signal light ofsaid optical variable attenuator; an output detecting unit which detectsan output level at an output side of said second optical amplifier; anoutput control unit which controls output lights of excitation lightsources of said first optical amplifier and said second opticalamplifier according to an output level detected by said output detectingunit; a gain inclination detecting unit which detects a gain inclinationof said second optical amplifier; and a gain inclination control unitwhich controls a gain inclination by adjusting the attenuation of anoutput signal light attenuated by said optical variable attenuatoraccording to a gain inclination detected by said gain inclinationdetecting unit.
 42. The optical amplifier apparatus according to claim41, wherein said gain inclination detecting unit branches awavelength-multiplexed signal light, and detects a gain inclination fromoptical signal levels of a shortest wave and a longest wave.
 43. Theoptical amplifier apparatus according to claim 41, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and,detects a gain inclination from optical signal levels ofthree or more waves.
 44. The optical amplifier apparatus according toclaim 41, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 45. Theoptical amplifier apparatus according to claim 41, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 46. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of the input signal light to said opticalamplifier; a first optical amplifier which amplifies the signal light;an optical variable attenuator which attenuates an output signal lightof said first optical amplifier; a second optical amplifier whichamplifies an output signal light of said optical variable attenuator; awavelength selecting unit which interrupts a compensation light at anoutput side of said second optical amplifier, and transmits only asignal light; an output detecting unit which detects an output level ofa signal light at an output side of said wavelength selecting unit; anoutput control unit which controls the attenuation of an output signallight attenuated by said optical variable attenuator according to anoutput level detected by said output detecting unit; a gain inclinationdetecting unit which detects a gain inclination of said second opticalamplifier; and a gain inclination control unit which controls a gaininclination by adjusting an output light of said compensation lightsource according to a gain inclination detected by said gain inclinationdetecting unit.
 47. The optical amplifier apparatus according to claim46, wherein said gain inclination detecting unit branches awavelength-multiplexed signal light, and detects a gain inclination fromoptical signal levels of a shortest wave and a longest wave.
 48. Theoptical amplifier apparatus according to claim 46, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels ofthree or more waves.
 49. The optical amplifier apparatus according toclaim 46, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 50. Theoptical amplifier apparatus according to claim 46, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 51. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of the input signal light to said opticalamplifier; a first optical amplifier which amplifies the signal light;an optical variable attenuator which attenuates an output signal lightof said first optical amplifier; a second optical amplifier whichamplifies an output signal light of said optical variable attenuator; awavelength selecting unit which interrupts a compensation light at anoutput side of said second optical amplifier, and transmits only asignal light; an output detecting unit which detects an output level ofa signal light at an output side of said wavelength selecting unit; anoutput control unit which controls an output light of said compensationlight source according to an output level detected by said outputdetecting unit; a gain inclination detecting unit which detects a gaininclination of said second optical amplifier; and a gain inclinationcontrol unit which controls a gain inclination by adjusting theattenuation of an output signal light attenuated by said opticalvariable attenuator according to a gain inclination detected by saidgain inclination detecting unit.
 52. The optical amplifier apparatusaccording to claim 51, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of a shortest wave and a longestwave.
 53. The optical amplifier apparatus according to claim 51, whereinsaid gain inclination detecting unit branches a wavelength-multiplexedsignal light, and detects a gain inclination from optical signal levelsof three or more waves.
 54. The optical amplifier apparatus according toclaim 51, wherein said optical amplifier apparatus inputs awavelength-multiplexed signal light having mutually different frequencycomponents superimposed on optical signals of a shortest wave and alongest wave respectively, and said gain inclination detecting unitdetects frequency components superimposed on the optical signals ofrespective wavelengths thereby to detect a gain inclination.
 55. Theoptical amplifier apparatus according to claim 51, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof three or more waves respectively, and said gain inclination detectingunit detects frequency components superimposed on the optical signalsthereby to detect a gain inclination.
 56. An optical amplifier apparatuscomprising: an optical amplifier which amplifies an inputwavelength-multiplexed signal light; a compensation light source whichinjects a compensation light that propagates in a forward direction of apropagation direction of the input signal light to said opticalamplifier; a first optical amplifier which amplifies the signal light; asecond optical amplifier which amplifies an output signal light of saidfirst optical amplifier; a wavelength selecting unit which interrupts acompensation light at an output side of said second optical amplifier,and transmits only a signal light; an output detecting unit whichdetects an output level of a signal light at an output side of saidwavelength selecting unit; an output control unit which controls outputlights of excitation light sources of said first optical amplifier andsaid second optical amplifier according to an output level detected bysaid output detecting unit; a gain inclination detecting unit whichdetects a gain inclination of said optical amplifier; and a gaininclination control unit which controls a gain inclination by adjustingan output light of said compensation light source according to a gaininclination detected by said gain inclination detecting unit.
 57. Theoptical amplifier apparatus according to claim 56, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.
 58. The optical amplifier apparatusaccording to claim 56, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of three or more waves.
 59. Theoptical amplifier apparatus according to claim 56, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.
 60. The optical amplifier apparatus according to claim 56,wherein said optical amplifier apparatus inputs a wavelength-multiplexedsignal light having mutually different frequency components superimposedon optical signals of three or more waves respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals thereby to detect a gain inclination.
 61. An opticalamplifier apparatus comprising: an optical amplifier which amplifies aninput wavelength-multiplexed signal light in an excitation light source;a compensation light source which injects a compensation light thatpropagates in a forward direction of a propagation direction of theinput signal light to said optical amplifier; a first optical amplifierwhich amplifies the signal light; a second optical amplifier whichamplifies an output signal light of said first optical amplifier; awavelength selecting unit which interrupts a compensation light at anoutput side of said second optical amplifier, and transmits only asignal light; an output detecting unit which detects an output level ofa signal light at an output side of said wavelength selecting unit; anoutput control unit which controls an output light of said compensationlight source according to an output level detected by said outputdetecting unit; a gain inclination detecting unit which detects a gaininclination of said second optical amplifier; and a gain inclinationcontrol unit which controls a gain inclination by adjusting outputlights of an excitation light source of said first optical amplifier andan excitation light source of said second optical amplifier according toa gain inclination detected by said gain inclination detecting unit. 62.The optical amplifier apparatus according to claim 61, wherein said gaininclination detecting unit branches a wavelength-multiplexed signallight, and detects a gain inclination from optical signal levels of ashortest wave and a longest wave.
 63. The optical amplifier apparatusaccording to claim 61, wherein said gain inclination detecting unitbranches a wavelength-multiplexed signal light, and detects a gaininclination from optical signal levels of three or more waves.
 64. Theoptical amplifier apparatus according to claim 61, wherein said opticalamplifier apparatus inputs a wavelength-multiplexed signal light havingmutually different frequency components superimposed on optical signalsof a shortest wave and a longest wave respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals of respective wavelengths thereby to detect a gaininclination.
 65. The optical amplifier apparatus according to claim 61,wherein said optical amplifier apparatus inputs a wavelength-multiplexedsignal light having mutually different frequency components superimposedon optical signals of three or more waves respectively, and said gaininclination detecting unit detects frequency components superimposed onthe optical signals thereby to detect a gain inclination.